I propose to investigate the developmental interaction between the trigeminal nerve and the morphogenesis of the craniofacial complex in the grass frog, R. pipiens. This system is of special interest due to the dramatic reorganization of the visceral cranium during metamorphosis. I want to learn how the central nervous system compensates for rapid developmental alterations of the jaws that occur at the time of transition from the larval to the adult state, and to investigate the possibility that the trigeminal neurons may provide cues that initiate cell death in the larval jaw muscles. Specifically, the aims of this proposal are threefold: 1) to document the magnitude, sequence and events associated with the degeneration and regeneration of the jaw muscles, 2) to detail the concomitant changes in the architecture and connectivity of the neurons of the trigeminal motor and mesencephalic nuclei, and 3) to record the physiologic activity of these neurons before, during and after the breakdown of the larval jaw muscles in order to determine if there are major alterations in the activity of these neurons at the onset of histolysis of the jaw muscles. The first objective will be achieved via a combination of radioautography, light and electron microscopic examination of the jaw muscles. These studies will focus on the birth dates of the larval and adult myoblasts and the structural changes in the larval muscle cells during cell death. During phase two the fate of the neuronal pools innervating the larval muscles will be analyzed using retrograde tracer techniques in combination with light and electron microscopy. These studies will concentrate on the dendrite architecture and synaptic connections of the trigeminal motoneurons. During phase three I will attempt to do electrophysiologic recordings of the trigeminal neurons during the period of cell degneration in the muscles that they innervate.